A personal name, symbol, letter, and photograph are applied on a plastic card such as a credit card, cash card, ID card, and tag card by silk printing or sublimation printing. However, there is a problem of inferior productivity because of a complex manufacturing process.
On the other hand, as a printing technique using no ink, there has been proposed a laser marking method where printing is performed on a plastic formed body by irradiating a laser beam in, for example, Patent Documents 1 to 5.
However, when a method where laser marking is taken into consideration is applied to a plastic multilayer sheet having a superficial transparent surface skin layer, various kinds of problems arise. For example, in the multilayer sheet of Patent Document 7, when adhesion between the surface skin layer and the inner layer is bad, the transparent surface skin layer is expanded or damaged by gas generating upon marking to cause a new problem if the conditions of laser marking on the inner layer serving as an energy adsorber of a laser beam do not sufficiently match.
Furthermore, when the multilayer sheet in Patent Documents 7 and 8 is used as an over-sheet for a card, there arise problems of sticking of the sheet to a carrier machine even if negative pressure is cancelled in a sheet-carrying step, sticking of the over-sheet on a die (die-sticking) in the step of thermal fusion bonding at the space between sheets by hot press, and expanding or damaging the transparent surface skin layer due to gas burned generating at the interface between the core sheet and the over-sheet when a laser beam is irradiated because outgassing is bad at the interface between the core sheet and the over-sheet. Thus, further improvement has been required.
[Patent Document 1] JP-B-61-11771
[Patent Document 2] JP-B-62-59663
[Patent Document 3] JP-B-61-41320
[Patent Document 4] JP-A-61-192737
[Patent Document 5] JP-B-2-47314
[Patent Document 6] JP-A-7-276575
[Patent Document 7] JP-A-2002-273832
[Patent Document 8] Japanese Patent No. 3889431